Electrical equipment or appliance connected to the AC power grid line should satisfy the current harmonic standard IEC61000-3-2. For different equipment or application, IEC61000-3-2 defines their corresponding current harmonic limits. Class A is for normal electrical equipment, Class C is for lighting equipment and class D is for portable personal computer, monitor and TV.
Current switching mode power supply technology, in order to realize the power factor correction function, mainly utilizes the active PFC method which adopts boost topology (FIG. 1 and FIG. 2).
Such boost converter integrates with a DC-DC converter and has an excellent power factor correction performance. Due to independent circuits of boost and DC-DC converters, individual power switching components, feedback control and driving units are used resulting in high cost, big size and low efficiency.
In order to overcome above shortcomings, a combination circuit of boost and bridge converters is presented in FIG. 3. This circuit, by sharing switching component of bridge type converter, drives boost inductor, saves boost switching component and boost rectifier in conventional boost PFC circuit, saves individual PWM PFC control unit, as the result, cost reduced, space saved and efficiency improved.
However, above combination power supply (FIG. 3), sharing common switching components, makes it impossible to control the boost converter and the DC-DC converter together by the conventional feedback control and the driving unit 300. Disadvantages are listed as following:
Disadvantage 1: The boost output voltage is not under control.
By using the feedback and the driving controller 300, the first switching component Q2 and the second switching component Q3 are driven by the PWM complement signal, so that the DC-DC output can be close-loop stabilized by adjusting the duty of PWM signal as convention. However, due to the same PWM signal to drive the boost circuit via the first switching component Q2, the boost circuit output has no feedback control and it swings according to the DC-DC's PWM and the input AC instant voltage. The bridge type DC-DC converter's maximum duty is always smaller than 50%, therefore, the boost converter's duty is also limited to less than 50%. Instead of conventional almost up to 100% operation in the boost converter, that 50% duty at the input AC low instant voltage makes the boost converter insufficient to convert the power, so that the boost output voltage on the storage capacitor C2 would be possible lower than the input AC peak voltage. Charging the current through the rectification circuit D1 might occur at the peak AC instant voltage as shown in FIG. 5A and FIG. 5B which makes the AC input current distorted at the low input voltage and heavy load.
Disadvantage 2: When the boost inductor L1 operates at the high AC input voltage, due to the low reset voltage Vdc−Vin (Vdc: boost output voltage on the storage capacitor C2), the unlimited duty decided by the DC-DC converter would cause the boost inductor insufficient magnetic reset, therefore Vdc has to be designed high enough to ensure resetting.
Vdc is the voltage on the storage capacitor C2 and it is the output voltage of the boost circuit as well. Vin is the voltage on the boost capacitor C1, which reflects the instant voltage of the AC input in real time. Duty means the duty cycle of the DC-DC converter. The magnetic reset equation of the boost inductor L1 is:Vin·Duty=(Vdc−Vin)·(1−Duty)
So Duty(Max)=(Vdc−Vin)/Vdc can be derived. When Vin at its sinusoidal peak, due to instant Vin close to Vdc, Duty(max) has to be very small to ensure the voltage-second balance to realize the magnetic reset. Once the DC-DC converter's duty is more than the Duty(max), the boost inductor L1 saturated and the second switching component Q2 damaged.
Disadvantage 3: When the boost inductor L1 operates at a continuous current mode, the first switching component Q2 works in a hard switching mode and the loss increased.
When the boost inductor L1 operates at a continuous current mode, the first switching component Q2 works in hard switching mode as that in convention boost converter. During the cut-off interval of Q2, there are no resonant current discharges its parasitic capacitor, so ZVS does not occur at its turning on.
Disadvantage 4: When the boost inductor L1 operates at discontinuous current mode, the boost inductor L1, the boost capacitor C1, the second switching component Q3 and the storage capacitor C2 construct a resonant tank by which results in an uncontrolled resonant loop loss.
When the boost inductor L1 operates at a discontinuous current mode, the boost inductor L1, the boost capacitor C1, the second switching component Q3 and the storage capacitor C2 construct a resonant circuit. After the boost current down to zero ampere, and at the state of the first switching component Q2 turning off and the second switching component Q3 turning on, the voltage on the storage C2 charges the resonant tank of the boost inductor L1 and the boost capacitor C1 through the second switching component Q3, to form the resonant current which cause loop loss if it is not under control.
For purpose of solving above problems, a PFWM (Pulse Frequency Width Modulation) method is disclosed to replace the conventional PWM control method.